1. Field of the Invention
The present invention relates to a liquid crystal composition, a color filter and an optical film using such a liquid crystal composition.
2. Description of the Related Art
In general, a color filter used for a color liquid crystal display, etc., is composed of, red (R), green (G) and blue (B) pixels respectively, and black matrixes that are formed between the pixels so as to improve display contrast. Conventionally, the color filter of this type has been mainly formed by dispersing a pigment into a resin or dyeing a resin with a dye. In the manufacturing method also, such a colored resin liquid is coated onto a glass substrate by spin coating, etc., to form a colored resist layer. This is patterned by photolithography to form a color filter pixel, or a colored pixel is printed directly on a substrate; thus, a color filter is manufactured. However, in the manufacturing method of a color filter using a printing method, there is a drawback in that the pixel resolution is low and it is difficult to produce a high-resolution image pattern. In the manufacturing method using the spin coat method, there is also a drawback in that there is a large amount of material loss. Further, it is generally necessary to carry out mask exposure three times in order to form R, G and B pixels respectively, and as a result the manufacturing process is complex and manufacturing cost is high.
The color filter is required to have properties of high transmittance and high color purity. In a method in which a dye is used, transmittance and purity may be improved by optimizing the type of dye and the color resin. Also, in a method using a pigment, transmittance and purity may be improved by using a fine pigment which is more thoroughly dispersed. However, since all of the conventional color filters are color filters of the light-absorbing type, there is a limit to how much the color purity can be improved by improving the transmittance. In recent years, there have been high demands for transmittance and color purity of color filters used, in liquid crystal display (LCD) panels. In particular, in reflection-type LCD color filters, it is very difficult to simultaneously achieve a degree of whiteness like white paper, contrast and color reproducibility, but there have been particularly high demands for color filters having these properties.
There have been ever-increasing expectations for color filters utilizing the light selective reflection of the cholesteric liquid crystal structure as color filters (hereinafter, sometimes, referred to as xe2x80x9clight reflection type color filterxe2x80x9d) that can solve the above-mentioned problems of the light-absorbing type color filters. In this light reflection type color filter, only light rays having specific wavelengths are reflected by utilizing Bragg reflection derived from circularly polarized light from a cholesteric liquid crystal, and RGB pixels are formed by using this function. Since the spectrum profile of the reflected light has a virtually square shape, it is possible to increase the color purity without causing a reduction in the transmittance. And since there is no light absorption like that of metal, it is possible to obtain a reflection rate of virtually 100% for circularly polarized light. Thus, this type of superior color filter was long awaited.
With respect to the manufacturing method of the reflection type color filter, a liquid crystal composition, which contains a cholesteric liquid crystal formed by coupling chiral units that isomerize upon irradiation with light, is coated by a spin coating method, etc., and light rays having different light intensities are irradiated to carry out patterning in a high-temperature liquid crystal state, and this is quickly cooled off so as to carry out fixing. The cholesteric liquid crystal achieves orientations having different cholesteric pitches in accordance with the light intensities of the rays irradiated for patterning. As a result the distribution of the cholesteric pitches forms pixels having colors of RGB respectively. Moreover, another manufacturing method for the reflection type color filter is one in which a liquid crystal composition, containing materials, such as a polymerizable nematic liquid crystal as a main component, a chiral compound which undergoes a structural change upon irradiation with light, a polymerizable monomer and a polymerization initiator, is subjected to a pattern irradiation in the same manner as the method described above, and is then further subjected to an irradiation of light having a different wavelength so as to be polymerized and thereby form a hard film. In these manufacturing processes, a patterning process for color filters of respective colors of RGB is achieved by carrying out a mask-exposure once. Therefore, as compared with the conventional processes requiring mask exposure to be carried out three times, it is possible to simplify the manufacturing processes. These methods are superior with respect to manufacturing speed, a reduction in the facilities required and material loss among other points. Moreover, optical anisotropic films manufactured by irradiating light onto the liquid crystal composition so as to fix the liquid crystal molecules are not only used as color filters, but also expected to be applied to other optical films such as optical compensation films used for display elements.
However, although the reflection type color filter has superior color purity, there is still some room for improvement of lightness. There is demand for the development of a reflection type color filters that make it possible to provide lighter display. Moreover, the optical film needs to have a certain degree of thickness in order for the liquid crystal molecules to be aligned with a predetermined orientation, and in order to exert a desired optical anisotropy. The optical film having this thickness is not small and thin enough for use as an optical compensation film and the like of a display element. Here, when a liquid crystal composition having a great birefringence xcex94n is utilized, it becomes possible to improve the reflectance of the color filter and also to provide a lighter display. Therefore, this method is advantageous, and also makes thinner optical films for various usages.
The present invention has been devised to solve the above-mentioned problems of the conventional devices, and its objective is to provide a liquid crystal composition whose orientation changes significantly upon irradiation with light, and also has a large birefringence xcex94n. Another objective of the present invention is to provide a reflection type color filter which has a high reflectance and provides a lighter display. Yet another objective of the present invention is to provide an optical film which can be made thinner.
One aspect of the present invention is a liquid crystal composition comprising at least one of compounds represented by the following formula (1) or formula (2) and at least one of chiral compounds whose structure change upon photoreaction: 
wherein, each of R1, R2, R3 and R4 represents at least one monovalent group selected from group 1 which consists of monovalent groups, each of L1 and L2 represents a single bond or at least one divalent group selected from group 2 which consists of divalent groups, each of Ar1, Ar3, Ar4 and Ar6 represents at least one divalent group selected from group 3 which consists of divalent groups, each of Ar2 and Ar5 represents at least one divalent group selected from group 4 which consists of divalent groups, and a carbon ring in groups 3 and 4 which consists of divalent groups may be substituted by at least one of a fluorine atom, a chlorine atom, a bromine atom, xe2x80x94CF3, xe2x80x94OCF3, xe2x80x94OCHF2, xe2x80x94CH3 and xe2x80x94COCH3; each of n1, n2, n3 and n4 represents 0 or 1, and n represents any one of integers from 2 to 15 and groups 1,2,3, and 4 are as follows: 
Another aspect of the present invention is a color filter, comprising a layer having colored areas which are formed to have different selective reflections by irradiating active light rays having different irradiation amounts onto the layer having a liquid crystal composition that includes at least one of compounds represented by the formula (1) or formula (2) and at least one of chiral compounds that undergo structural changes upon photoreaction:
Yet another aspect of the present invention is a color filter, comprising a layer having a red area, a green area and a blue area, the respective areas showing colors due to circularly polarized reflections caused by helical pitches with which a compound represented by the formula (1) or formula (2) is oriented and fixed.
A further aspect of the present invention is an optical film, which is formed by irradiating active light rays onto a layer containing a liquid crystal composition that includes at least one of compounds represented by the formula (1) or formula (2) and at least one of chiral compounds that undergo structural changes upon photoreaction to cause a compound represented by the formula (1) or formula (2) to be polymerized and fixed.